Comparison of Delivery Methods for Enhanced In Situ Remediation in Clay Till

Abstract

Three methods for enhanced delivery of in situ remediation amendments in low‐permeability deposits have been tested at a site in Denmark: pneumatic fracturing, direct‐push delivery, and hydraulic fracturing. The testing was carried out at an uncontaminated part of a farm site, previously used for storage of chlorinated solvents, underlain by basal clay till with hydraulic conductivity ranging from 7.1× 10–11 to 3.5 × 10–7 m/s at testing depths 2.5 to 9.5 m b.s. Fluorescent tracers fluorescein and rhodamine WT were delivered. Tests of all three delivery methods have not been carried out at a single site before, and thus, this study provides unique data for comparison of enhanced delivery methods in both the vadose and saturated zone. Results show that pneumatic fracturing with nitrogen gas and propagation pressures of 1 to 9 bar had a distribution radius of less than 2 m, and produced dense networks of tracer‐filled natural fractures above the redox boundary (0 to 3 m b.s.) and widely spaced, discrete, induced, tracer‐filled subhorizontal fractures at depth (>3 m b.s.). Direct‐push delivery at pressures of 8 to 30 bar had a distribution radius of approximately 1 m, distributed tracer primarily in natural fractures above the redox boundary and in discrete, closely spaced (but not merging) induced fractures below the redox boundary. Hydraulic fracturing with a sand‐guar mixture at pressures of 0 to 6 bar produced an elliptical, asymmetrical, bowl‐shaped fracture with a physical radius of approximately 3.5 m at 3 m b.s. The geometry of hydraulic fractures attempted emplaced at 6.5 and 9.5 m b.s. is uncertain, but clearly not horizontal as desired. The direct‐push delivery method is robust and efficient for enhanced delivery at the clay till site in question, which based on thorough geological characterization is deemed a geologically representative basal clay till site.

Insights from years of performance that are shaping injection‐based remediation systems

“…High-pressure injection typically propagates reagent via very thin (millimeter-size) fractures (Christiansen et al, 2010).…”

“…Injection by this method has been shown to achieve a relatively small effective distribution radius (1 meter) and a large variation in achievable delivery volume between injection points (Christiansen et al, 2010). It can also lead to short-circuiting of reagent solution to land surface or the vadose zone, where it does not provide value.…”

“…This manner of injection-without fracturing-is ideal to obtain reagent distribution and subsequent treatment within the targeted treatment area. If fracturing is needed, it must be approached carefully and adequate vertical distribution verified (Christiansen et al, 2010).…”

A Long-Term Field Study of In Situ Bioremediation in a Fractured Conglomerate Trichloroethene Source Zone

“…The conglomerate at this site is also highly indurated (i.e., very hard), and direct push technology previously attempted at this site was unable to penetrate it. Thus, the strategy often used at lowerpermeability sites, installing a large number of wells to implement bioremediation (Damgaard et al 2013), fracturing methods (Strong et al 2004;Christiansen et al 2010;Scheutz et al 2010), or high-resolution sampling (Takeuchi et al 2011), is impractical because direct push technology cannot be used at this site.…”

ZVI-Clay remediation of a chlorinated solvent source zone, Skuldelev, Denmark: 1. Site description and contaminant source mass reduction

“…In addition, the build-up of contaminant mass by matrix diffusion into low-permeable geological layers holds a subsequent risk of post-treatment back-diffusion (Chapman and Parker, 2005;Liu and Ball, 2002;VanderKwaak and Sudicky, 1996). Especially the injection based remediation technologies are restricted by subsurface heterogeneities as mixing at low permeability sections of the contaminated subsurface is very challenging (Christiansen et al, 2010). These technologies include: in situ chemical oxidation; in situ flushing with cosolvents or surfactants; and enhanced bioremediation by biostimulation or bioaugmentation (Jawitz et al, 2000;Krembs et al, 2010;McGuire et al, 2006).…”